(1) Field of the Invention
The present invention relates to a high frequency dividing apparatus able to divide an input signal belonging to a microwave band.
(2) Description of the Related Art
Recently, the development of communication systems has allowed vehicles to be provided with a mobile communication system which can be used as a telephone system and a satellite communication system. An automobile equipped with a mobile communication system, e.g., a mobile radio telephone system, is called a mobile station, and this mobile station can communicate not only with a fixed telephone such as a telephone in an office or at home but also mobile stations. This communication is established by radio waves between the mobile station and a land station, and transferred to a conventional telephone system at the land station. The satellite communication system is also established by radio waves between the earth station and the satellite station.
A frequency dividing apparatus to be used in a synthesizer of a communication system such as the mobile communication system or the satellite communication system, must be able to generate various frequencies by dividing an input frequency in a high frequency band, such as a microwave band of approximately from 1 GHz to 30 GHz. Namely, the frequency dividing apparatus in a satellite communication system must divide higher frequencies than those divided by a conventional frequency dividing apparatus.
Known conventional frequency dividing apparatuses are composed of logic circuits, such as flip-flops. FIG. 1 is a typical prior art frequency dividing apparatus composed of flip-flops FF1, FF2, and FF3, and a gate circuit G1. In FIG. 1, the flip-flops FF1, FF2, and FF3 are D-type flip-flops having an input D, input C, and output Q (only FF3 has an input M), and the gate circuit G1 is an OR-gate. Each of the flip-flops FF1, FF2, and FF3 is a 1/2 frequency divider. The output Q of the flip-flop FF1 is connected to the input D of the flip-flop FF2, the output Q of the flip-flop FF2 is connected to the input D of the flip-flop FF3 and one input of the OR gate G1, and the output Q of the flip-flop FF3 is connected to the other input of the OR gate G1. Namely, the outputs Q of the flip-flops FF2 and FF3 are fedback to the input D of the flip-flop FF1 through the OR gate G1. The inputs C of the flip-flops FF1, FF2, and FF3 are used as a common clock signal to be divided, and the inputs D are used for data or as delay signals. The input M of the flip-flop FF3 is used as a control signal and an output divided from the output Q of the flip-flop FF1 is made 1/4 or 1/5 by the control signal.
In the frequency dividing apparatus shown in FIG. 1, a 1/4 divided output frequency or a 1/5 divided output frequency can be selected by halting the operation of the flip-flop FF1 intermittently, by controlling the timing of a signal input to the input M of the flip-flop FF3. That is, when the flip-flop FF1 is reset to stop the dividing operation, at a suitable timing obtained by counting the number of clocks, the clock frequency is divided into 1/5 and appears as a divided output of the flip-flop FF1. When the flip-flop FF1 is activated to start the dividing operation, the clock frequency is divided into 1/4 and appears as a divided output of the flip-flop FF1.
In the conventional frequency dividing apparatus as described above, the clock signal input in the flip-flop FF1 is returned through the flip-flop FF2, FF3 and the gate G1 with a certain delay time, which depends upon the number of gates the signal must pass through. This limits the speed at which the frequency is divided in the dividing apparatus. In other words, when the dividing speed of the dividing apparatus is low, the dividing apparatus can not divide high frequencies. The prior art frequency dividing apparatus shown in FIG. 1, which can be used in the mobile station of the mobile communication system, is the fastest known frequency divider, but naturally has a certain delay time as described above. Therefore, the maximum dividable frequency of the prior art frequency dividing apparatus is approximately 2.3 GHz. A frequency dividing apparatus which can divide a frequency of more than 2.3 GHz has not been realized until now.